The significant increase in LDH activity at maximum
levels of MPs concentrations can be explained by the occurrence of cellular
damage in organs and tissues due to anoxic situations during stressful
conditions (Abhijith et al., 2016), as previously identified in E. andrei with histopathological damage
(Rodríguez-Seijo et al., 2017). These results
were also in agreement with other studies that assessed the depletion of energy
reserves and LDH levels among marine worms (Wright et al., 2013), mussels
(Paul-Pont et al., 2017) and terrestrial organism, such as mice (Deng et al.,
2017) when exposed to different polymers.When
exposed to several contaminants, earthworms are highly susceptible to oxidative
stress stemming from the peroxidation of lipid tails in the membrane structure,
due to their high content of polyunsaturated fatty acids (Markad et al., 2012;
Mwaanga et al., 2014). Organisms exposed to
higher MPs concentrations (500 and 1000 mg kg -1) had the highest
mean values of TBARS per mg protein, with statistically significant differences
when compared with control earthworms and those exposed to other concentrations
(ANOVA: F = 70.73; d.f. 5, 48; p = 0.000) (Figure 2). At the same time,
at the lowest level of MPs concentrations, the lowest TBARS content was observed.These results indicate that the
lowest MPs levels activated the
earthworms’ defensive systems to prevent excess ROS production, as
indicated by Paul-Pont et al. (2016) for marine mussels exposed to polystyrene
microplastics. On the other hand, the increase
of TBARS levels in E. fetida may have
resulted from the incapacity of the defence system to counteract oxidative
stress in earthworms exposed to the highest concentrations of MPs (r = 0.668; p < 0.01) (Table 1) (Lin et al., 2012;
Zhang et al., 2013; Paul-Pont et al., 2016; Babic et al., 2016; Chao et al.,
2016). Oliveira et al. (2013) also detected no damage to lipid
peroxidation when P. microps were exposed to polyethylene
microspheres, although at lower MPs concentrations than those herein tested (in
their case, 200 ug L-1 and 1-5 µm). In this
sense, the biomarker data collected in the present study are complementary to
the histopathological and molecular analyses conducted in the previous study,
as the incrementing concentrations of lipids, proteins and polysaccharides can
occur after lipid peroxidation (García et al., 2005) or through coelomocytes
involved in eliminating foreign material (Kauschke et al., 2007).
According to Deng et al. (2017), MPs exposure
may cause inflammatory responses, which were also observed in our previous research,
and thus lead to lipid disturbance, as reported for mussels (von Moos et al., 2012),
or fishes (Oliveira et al., 2013). The absence of mortality or
significant changes in weight suggested that antioxidant enzymes may play a key role in the health status of the organism
and can prevent damages at several levels. However, the antioxidant response
was insufficient to hold the oxidative stress from exposure to MPs (Markad et
al., 2015), as was previously observed through histopathological and molecular
changes in E. andrei (Rodríguez-Seijo
et al., 2017).